added texture to gfx; fixed gl includes

modules/gfx/pymod/wrap_gfx.cc

modules/gfx/src/CMakeLists.txt

modules/gfx/src/bitmap_io.cc

modules/gfx/src/gfx_test_object.cc

modules/gfx/src/gfx_test_object.hh

modules/gfx/src/gl_helper.hh

modules/gfx/src/map_slab.hh

modules/gfx/src/scene.hh

modules/gfx/src/shader.hh

modules/gfx/src/texture.cc

modules/gfx/src/texture.hh